Aromatic polyimides are known to exhibit exceptional mechanical and thermal properties in continuous use above 200.degree. C. These polymers have been considerably developed as insulating films or varnishes of remarkable thermal stability (at least 1 000 hours at 250.degree. C.), good flexibility and very good resistance to thermal shock and whose thermo plasticity temperature generally ranges from 400.degree. to 500.degree. C.
Most of the aromatic polyimides are also known as being insoluble and their use infusible and requires techniques which either need a non cyclized soluble polyamide-acid intermediary or a mixture of fusible reacting monomers which, by thermal treatment, may give a polymeric material.
According to a first technique, the polyimide is prepared in two successive steps. During the first step, a reaction between an aromatic diamine and a dianhydride of tetracarboxylic acid in an aprotic polar solvent gives a solution of polyamide-acid of high molecular weight. As the properties of the final polyimide (flexibility, thermoplasticity and thermal stability) are the proportional to the molecular weight, it is imperative to use very pure solvents and reactants. Moreover, the amounts must be used in stoichiometrical proportions. The polyamide-acids of high molecular weight however give very viscous solutions, thus limiting the concentration of dry material to values ranging from 10 to 15% by weight. In spite of this low concentration, the dynamic viscosity of the solutions easily reaches 20 000 to 50 000 mPa.s.
For use as protective layer, where the thickness of the polyimide coating is a few microns, a low polymer concentration is not redhibitory, but when a deposit of several tens of microns must be obtained it is necessary to use numerous passes through the coating solution to obtain the desired thickness.
The second step of the manufacturing process consists of using a solution containing generally 10 to 15% by weight of polyamide-acid for preparing a polymer film by progressive evaporation of the solvent. The conversion of the polyamide-acid to a polyimide film is obtained by thermal or chemical dehydration.
This general technique for manufacturing polyimides is disclosed in particular in the French Pat. Nos. 1,239,491 and 1,256,203. In the second of said patents an example describes the use of polyimides as enamelling varnishes for electric wires. According to said example, pyromellitic dianhydride is reacted with bis-(p-amino-phenyl)ether dissolved in dimethylacetamide, to prepare a polyamide-acid having an inherent viscosity of 1.1 dl/g. For being usable on a coating machine, the polymer concentration is reduced to 11% (by weight) and ten successive posses through the enamelling solution are necessary to obtain an increase of the wire diameter of less than 50 microns.
Another method known for preparing aromatic polyimides consists of replacing the dianhydrides of tetracarboxylic acids by the products of their reaction with primary aliphatic alcohols, i.e. by the corresponding alkyl diesters or tetraesters. This polycondensation technique is disclosed in the French Pat. No. 1,360,488 and by V. L. Bell (Polymer Letters, 1967, 5, 941-946) for alkyl diesters of pyromellitic acid and 3,3',4,4'-benzophenone tetracarboxylic acid and various aromatic diamines. The monomers mixture is converted to polyimides by progressive heating up to 275.degree. or 300.degree. C. Since, after solvent evaporation, the polycondensation takes place in solid phase, this method is mainly used to prepare adhesives, composite materials or impregnation varnishes, since the polyimides obtained have generally a rather low molecular weight and a low flexibility or no flexibility at all.
With alkyl diesters of the above-mentioned acids, it is not possible to achieve a polycondensation in solution, since the polyimides are obtained as oligomers of low molecular weight and are infusible and insoluble in the polymerisation solvents, as shown in the comparative examples of the French Pat. No. 2,514,772.
The advantages and disadvantages of the two techniques for preparing polyimides according to the prior art may be summarized as follows:
the technique using dianhydrides is adapted for obtaining high molecular weights but the solution concentration is low, this being a disadvantage for applications as films or as protective layers, PA1 the technique using diesters or tetraesters leads very quickly to insoluble products and it is impossible, according to the prior art, to obtain products of high molecular weight in solution. PA1 at least one dianhydride of tetracarboxylic aromatic acid of general formula ##STR1## at least one biprimary aromatic diamine of general formula: EQU NH.sub.2 --Ar'--NH.sub.2 ( 2) PA1 and at least one aromatic compound of the general formula: ##STR2## PA1 --O--; --S--; --SO--; --SO.sub.2 --; --CO--; --CHOH--; --CH.sub.2 --; --CF.sub.2 --; --C(CH.sub.3).sub.2 --; --C(CF.sub.3).sub.2 --; --COO--; --CONH--; --CO--O--(CH.sub.2).sub.x --O--CO--; --Si(CH.sub.3).sub.2 --; --O--Si(CH.sub.3).sub.2 --O--. PA1 Ar' is a carbocyclic or heterocyclic divalent aromatic radical, the two valencies of which are on separate carbon atoms, not in ortho or peri position with respect to each other. Radical Ar' may be formed of one or more rings coupled or linked together as above defined for radicals Ar and Ar". PA1 R is a hydrogen atom or a hydrocarbon monovalent radical, preferably containing 1 to 13 carbon atoms. In other terms, the compound of formula (3) may be a tetracarboxylic aromatic acid, or a bis(ortho-acid-ester) derived from a tetracarboxylic aromatic acid. In the latter case, the methyl diester is preferred.